The present invention relates to a method and to a device for producing a fiber composite component as well as to a fiber composite component.
It is known to produce fiber composite components from two or more layers of unidirectional or multi-directional long fiber arrangements, which hereafter in the present application are referred to as long fiber layers, wherein the long fiber layers are arranged crosswise or with arbitrary angular offset. In other words, multiple long fiber layers, in particular unidirectional (UD) layers, are stacked on top of each other with different orientations of the fibers, so as to prepare a preform or a prepreg, which is subsequently pressed into shape, wetted with a matrix (such as thermosetting or thermoplastic resins), impregnated and cured. An exemplary method for producing a fiber composite component made of at least two long fiber layers according to the prior art is schematically shown in FIG. 6.
According to the illustration in FIG. 6, initially a stack of long fiber layers 32, each having a defined fiber orientation, is prepared in a laminating station 31. The long fiber layers 32 are prepared from rovings (fiber strands made of quasi-continuous individual filaments), for example. In a preforming process 33, the long fiber layers 32 are then formed to obtain a preform 34. In an insertion step 35, the preform 34 is inserted into a mold 36 having a lower part 37 and an upper part 38, each having shaping surfaces that correspond to the shape of the component to be produced. The inserted preform is positioned for this purpose via the mold contour. If positioning via the mold contour is not possible, spring-loaded positioning pins (not shown) are used on the mold 36, for example. The preform is punched out, for example, and then positioned with the punched holes over the pins. The spring-loaded pins are later pushed by the upper part 38 into the lower part 37 when the mold is being closed. In a pressing process 39, the preform 34 is then pressed in the mold 36. This is done by what is known as an index closing process, for example. Initially, a gap of approximately 0.3 mm is set, which is to say the cavity between the upper part 37 and lower part 38 is closed, leaving an oversize of approximately 0.3 mm. The mold is then evacuated by way of venting elements and a vacuum of approximately −0.8 bar. This is intended to prevent air pockets and thus to decisively influence the quality of the RTM component. The venting time varies depending on the mold size and shape. A resin system 41 is then pressed in via an injector 40, said system saturating the preform 34 and forming a bond with the long fiber layers 32. The resin system can be a two-component epoxy resin (2K-EP) system, for example. The injector comprises a mixing head, for example, which is attached to the mold gate point. The individual components are atomized at approximately 110 bar in the mixing head. During injection over a shot time of up to 60 seconds, the internal pressure in the mold can rise to more than 80 bar. After injection has ended, the mold 36 is closed completely, which is to say the gap is reduced from approximately 0.3 mm to zero. Known as gap injection, this process allows the pressure during injection to be lowered, the component surface to be improved, and shrink marks and pores on the finished RTM component to be reduced. The curing time begins after the shot has ended. This time varies as a function of the resin system and mold temperature (approximately 100° C. with base resin, 125° C. with potential resin), and on average is 3 to 6 minutes. The temperature is controlled by way of heating circuits in the mold and related temperature control devices. After the curing time has passed, the finished component 43 is removed from the mold 36 in a demolding process 42. This means that the mold 36 opens automatically, and the ejector pins are actuated. The finished, cured component 43 is thus ejected from the lower part 37. After demolding, the component 43 can be removed either manually or automatically. The mold 36 is then cleaned by way of compressed air.
The above-described process is referred to as a resin transfer molding (RTM) process, for example. Due to the short curing time of an adhesive used in the resin system, this method can result in certain material characteristics, such as pores or the like. During use of the finished component 43, these may create stress peaks and form the starting points for cracks. If a crack forms within a roving, which is to say the reinforcement fiber arrangement, the crack may be stopped by the fibers. In the matrix, however, a crack may propagate freely in some circumstances. This may result in structural failure of the component, for example in the form of delamination.
Similar problems can also occur with liquid press methods, in which the long fiber layers 32 are impregnated with a fiber matrix already prior to shaping so as to form what is known as a prepreg, and the prepreg is pressed directly into shape, optionally without a preforming process 33 and/or pressing of the resin system 41 into the mold.
According to another known method used to produce pressure vessels comprising an Al liner and CFRP winding (wet winding), the finished pressure vessel is subjected to an autofrettage process to increase the service life of the liner. In the autofrettage process, the pressure vessel provided with the liner is subjected to overpressure exceeding the operating overpressure and the yield point of the liner, which causes the liner material to become reinforced due to the resulting (partial) plasticization of the liner and the subsequent reduction of inherent compressive stresses when the pressure is relieved. However, a plurality of microcracks may develop in the matrix, which can reduce the bursting strength.
Components made of multi-layer fiber composite material in general tend to fail due to delamination under load because cracks can propagate freely between the (unidirectional) layers (along what are known as pure resin regions). To ensure reliability, the components are therefore conventionally dimensioned with a large safety factor, whereby an inherent lightweight potential cannot be fully met.
It is an object of the present invention to create a method and a device for producing a fiber composite component as well as a fiber composite component, which avoid the disadvantages of the prior art. It is in particular an object of the present invention to make it possible to increase the service life of CFRP structures, or of fiber composite structures in general, at a lower component weight. It is another object to achieve a reduction in delamination in the case of excessive loading of a fiber composite component. Yet another object of the invention is to achieve an increase in the fracture strength of fiber composite components, for example with respect to impact-related damage. A further object of the invention is to create a possibility to use recycled material in the production of fiber composite components, and thereby enable lower material costs. Finally, it is an object of the invention to allow a reduction in the material expenditure, and thereby greater utilization of the lightweight construction potential of fiber composite structures, in particular CFRP structures.
The above-mentioned object(s) is/are achieved at least in partial aspects by a method according to embodiments of the invention, by a device according to embodiments of the invention, and by a fiber composite component according to embodiments of the invention. Features and details that are described in connection with the method according to the invention also apply in connection with the device according to the invention and the fiber composite component according to the invention, and in each case conversely and reciprocally, so that mutual reference is made, or can be made, in each case to the individual aspects of the invention with respect to the disclosure.
The invention is based on the consideration that a crack bridge made of a thin layer of short fibers, which is applied between two long fiber layers, is able to prevent the linear growth of a crack in the matrix because the crack is forced to take an energy-intensive detour, and additionally must pull the fiber out of the matrix. This results in an increase in the energy that is needed for the crack to grow (larger generated crack surface), crack propagation is slowed, and as a result the service life and static strength are increased.
According to a first aspect of the present invention, analogously a method for producing a fiber composite component made of at least two long fiber layers is provided, wherein the method comprises the following acts:                a) providing a long fiber layer;        b) applying short fibers to the long fiber layer; and        c) applying a further long fiber layer to the long fiber layer provided with the short fibers.        
The short fibers are preferably designed and dimensioned in such a way, and are applied such that a propagation of cracks in one of the long fiber layers into the respective other of the long fiber layers and/or delamination between the long fiber layers is made more difficult. Short fibers shall be understood to mean, in particular, fibers having a length that is small in relation to a length of long fibers of the long fiber layers. Ideally, the short fibers have a length of at least 0.5 mm, and the length thereof is limited to a maximum of 30 mm, preferably to a maximum of 10 mm, and in particular to a maximum of 3 mm. The majority of the short fibers that are applied is preferably in the indicated size range. Depending on the type of production of the short fibers, as is described hereafter in more detail, it is possible for individual short fibers to exceed the indicated length range; however, this does not cause any harm as long as a sufficient portion of the short fibers is within the indicated length range. In the broader sense, the term ‘short fibers’ comprises any elongate structure, such as very short fibers in the micrometer range, or also nanotubes or cut nanotubes, provided they exhibit the effect described above. Depending on the arrangement and fill level of the long fibers in the long fiber layers, it is possible to achieve optimal spreading in the indicated size ranges in such a way that the short fibers slow down cracking without acting as notches, so that it is possible to optimize the added weight resulting from the short fibers in relation to the increase in strength that is achieved. Further method steps, such as impregnation, preforming, pressing, curing and demolding, correspond to the method steps of known methods.
The use of the method according to the invention is not limited to two long fiber layers, but can be expanded to any arbitrary number of long fiber layers, comprising in each case an interposed layer of short fibers, if the above-described steps b) and c) are alternately carried out multiple times. It goes without saying that, when step b) is repeated, the further long fiber layer applied in a step c) is the long fiber layer mentioned there, to which then further short fibers are applied.
Particularly good interlocking of the long fiber layers can be achieved when the short fibers are applied in the above-described step b) in such a fashion that the short fibers at least partially penetrate into the long fiber layer, and wherein the further long fiber layer in step c) is applied in such a fashion that the short fibers at least partially penetrate into the further long fiber layer. In this way a separation of the long fiber layers can be effectively prevented. If steps b) and c) are carried out repeatedly, it goes again without saying that the long fiber layer, when step b) is carried out for the first time, is the long fiber layer provided in step a); however, it is the further long fiber layer that has been applied during a prior instance of carrying out step c) if step b) is carried out again.
In a preferred refinement of the method according to the invention, the short fibers are applied in step b) in such a fashion that the short fibers have at least one substantially random orientation. This may mean in particular that the short fibers are applied in a deliberately randomized manner. Due to the randomized orientation, no preferred crack propagation direction is able to develop. In other words, the crack must always look for a new path, which further increases the energy necessary for the crack to grow, and thus further slows crack propagation and further increases the service life and static strength of the component.
In a preferred embodiment of the method according to the invention, the short fibers are produced by cutting a fiber strand to size and/or by processing, in particular comminuting, preferably grinding or shredding, recycled material, in particular production scrap. A fiber strand shall be understood in particular to mean what is known as a roving, which can be continuously fed in the method. The use of a fiber strand has advantages with respect to storage and handling, in particular feeding, and the short fibers can in particular be produced directly and continuously prior to use. The use of a recyclate, which can also be combined with the use of fiber strands, may make it possible to reuse scrap materials. This allows raw materials to be saved and process optimization to be achieved, and optionally statutory requirements to be met. Statutory requirements may relate to the mandatory compliance with a certain recycled portion when using plastic materials, for example. This may also relate in particular to the appliance and vehicle industry, which may be obligated to accept old product returns. The method according to the invention may therefore also make a contribution to the reuse of accepted returned products and to a reduction in the amount of waste.
In an alternatively preferred embodiment of the method according to the invention, the short fibers are sprayed on, preferably together with a binder. In this way, applying and impregnating the short fibers can be combined in a single method step. Alternatively, the short fibers can also be sprinkled on.
In a further preferred embodiment alternative, the short fibers are applied as a textile sheet (for example, non-woven fabric or laid scrim) to the long fiber layer. This enables particularly simple feeding and application, which can also speed up the production cycle. It is furthermore possible to cut a non-woven fabric or laid scrim to size in advance, which can further speed up the production of the fiber composite component.
Even though some embodiments above were mentioned as alternatives, the idea of the invention also covers the optional combination of these embodiments.
In a preferred embodiment of the method according to the invention, the short fibers are impregnated with a binder prior to being applied. The impregnation can also take place at least partially while the short fibers are being applied, which can be carried out by way of spraying, for example. If textile semi-finished products and rovings are used, the impregnation can also be carried out after the individual filaments of the roving have been spread, for example. Impregnating the short fibers can prevent adhesion of the long fiber layers in the semi-finished fiber product (the preform), which can further facilitate handling.
Even though the long fiber layers can generally also be provided as a dry semi-finished product, it is also possible in one preferred refinement of the method according to the invention to impregnate the long fiber layers with a binder. A preform can thus be produced, which due to the action of the binder has dimensional stability for further processing steps. Preferably after all long fiber layers have been applied, a layer stack, which is formed by the long fiber layers comprising interposed short fibers in each case, can be pressed in a mold.
Matching the material to the long fiber layers and the matrix, or a potentially present fiber size, is particularly advantageous if the short fibers are made of the same material as the long fiber layers. This may also result in synergies in the process control (impregnation, matrix, and the like) of long fibers and short fibers.
The method is particularly suited for components made of carbon-reinforced plastic material. This means that, in one preferred refinement of the method according to the invention, the short fibers and/or the long fiber layers are produced at least substantially of carbon. However, other fiber/matrix combinations as well as fiber ceramics can also be advantageously influenced by the method according to the invention.
The method exhibits the advantages thereof in particular when the long fiber layers are unidirectional long fiber layers. The problem of delamination and crack propagation is particularly virulent in fiber composite components made of UD fiber layers. However, the method can generally also be employed when using MD layers, and such a use is covered by the invention.
In a further aspect, the invention also relates to a device for producing a fiber composite component made of at least two long fiber layers, wherein said device is designed to carry out the above-described method.
According to a further aspect of the present invention, a fiber composite component having a layer arrangement made of at least two long fiber layers is provided, in which the layer arrangement comprises an admixture of short fibers made in particular of the same material as the long fibers, wherein the short fibers are preferably provided in each case in a transition region between two long fiber layers and are preferably designed, dimensioned and arranged in such a way that a propagation of cracks in one of the long fiber layers into a respective adjacent long fiber layer is made more difficult. A component within the meaning of the invention shall be understood to mean both a finished component and an intermediate product or a semi-finished product. The fiber composite component in particular comprises UD layers, which are preferably made of long carbon fibers. The fiber composite component designed according to this aspect has the advantages and effects described above with respect to the method according to the invention.
In a preferred refinement of the fiber composite component according to the invention, the short fibers penetrate at least partially into one or both of two adjacent long fiber layers. In a further refinement of the fiber composite component according to the invention, the short fibers have an at least substantially random orientation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.